Take me to the river

Climate news doesn’t just turn up in an RSS feed; sometimes it jumps right into your lap. A day or two ago, I discovered that a new paper about sea temperatures around New Zealand during the Eocene (50 million years ago) has significant implications for climate modelling and is based on fieldwork done very close to my home. Stuff reports:

Using sedimentary rocks from the bed of the Waipara River in North Canterbury, an international research group led by GNS Science palaeontologist Chris Hollis has reconstructed ancient sea temperatures. They found surface sea water exceeded 30 degrees Celsius, and water at the sea floor hovered around 20ºC during an episode of greenhouse gas-induced global warming that lasted for between two million and three million years. “These temperatures are at the extreme end of modern tropical water masses,” Dr Hollis said. Year-round sea surface temperatures of 25ºC to 30ºC are today found only at the equator.

At the time, New Zealand was much closer to the South Pole — below 50ºS — yet supported tropical flora and fauna. (GNS press release here).

The Eocene and the associated Palaeocene Eocene Thermal Maximum (PETM) are interesting to climate scientists because at the time the world was in a warm greenhouse phase, and the PETM saw a burst of rapid additional warming associated with the injection of massive amounts of carbon into the atmosphere (possibly methane from hydrates). Modeling the climate of the time poses a few problems, however. Temperature reconstructions show that the Arctic was very warm (ice free, with crocodiles) but it’s difficult to get the models to reproduce that without driving tropical temperatures up to over 40ºC — and that should have caused a massive die-off that is not (yet) seen in the fossil record. New Scientist had a very good article discussing Eocene climate reconstructions earlier this year (not behind a paywall, thankfully).

The new NZ work confirms that Eocene NZ was much warmer than previously thought, so Hollis and his co-authors make the following point:

To us, this suggests that the earth’s climate is much more sensitive to changes in greenhouse gas levels than is allowed for in climate models, which appear to be underestimating the degree of warming. Extreme greenhouse warming either caused high latitudes to warm far more than low latitudes or equatorial regions warmed beyond the limits of life: above 40ºC.

Another piece in the jigsaw puzzle of climate reconstructions for the period, made the more interesting by coming from my back yard.

I live on a small farm above1 the Waipara River where Hollis and his team conducted their fieldwork. This section of the Waipara is world famous in New Zealand (to palaeontologists and geologists, at least) because of the fossils found here — marine dinosaurs, penguins, sharks and concretions much like the Moeraki boulders (click on the thumbnail to see “God’s Marbles”, a few km upstream from us) — as well as the extensive faulting2. Generations of geologists from the University of Canterbury have done fieldwork or enjoyed field trips here. It’s also scenically beautiful, and I’ve become involved in efforts to protect and enhance the whole river environment.

The Hollis et al paper was given advance publicity to draw attention to a conference, Climatic and biotic events of the Paleogene in Wellington, which includes a public Greenhouse Earth Symposium at Te Papa on Jan 14th. I’d love to be there, but when I contacted Chris to discuss the paper, I stumbled on the next best thing: the pre-conference field trip is visiting my neck of the woods on Tuesday. My chance to meet the experts, while they examine “the siliciclastic mid-Waipara section, containing a bioturbated K/T boundary and an Early Eocene record of tropical conditions”. I’m looking forward to it (thanks for the invite, Chris).

Finally, while poking around on the conference web site, I stumbled on the following fine example of geological humour in the guidelines for formatting abstract submissions:

What I did in my holidays

A. Geologist and W.E.T. Field-assistant

We went out in the pouring rain and freezing wind, dodging mad sheep, to find some rocks. When we did, I hit them with my big hammer. From this we deduced that we needed more money for a big expensive machine that goes ‘ping’ in order to obtain some real data. The rest of the year was spent writing (unsuccessful) grant proposals.

Literally above. Our house is on top of a 40m limestone cliff above the river. [↩]

The cliff is the result of the Boby’s Stream fault, a large active fault that ruptures roughly every 900 years, generating movement of up to 2.5m and quakes large enough to cause severe damage to our nearest city, Christchurch. The last quake was about 300 years ago. Our vineyard straddles the fault – hence our wine being called The Faultline Pinot Noir (first production to be bottled in February). [↩]

Which reminds me of a Freddy Trueman joke. During commentary on a test match, someone made mention of the ample proportions of the backside of a fast bowler. Freddy offered, in his broad Yorkshire accent, “Aye, a big nail needs a big hammer”. [↩]

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7 thoughts on “Take me to the river”

Kerry Emanuel proposed a few years ago that the solution to the too-hot-paleo-tropics problem was increased tropical cyclone activity. He and others (notably Matt Huber) have done considerable work on this, but the lack of any sort of proxy for such activity leaves things a little open-ended (although to my knowledge no other mechanism has been proposed). Modeling work on this hypothesis continues, but the obvious point is that we should expect to see evidence of something like this kicking in as the planet heats up. Whether we should be seeing it this soon is also an open question, but do the fresh results showing increasing tropical convective cloud activity fill the bill? (Sorry, no links for the moment since I’m in a hurry.)

(Something strange seems to be happening to your formatting, BTW. The text in the comment box got smaller and there are run-together lines toward the bottom of the post. This is with IE.)

Here is a recent article by KE surveying the present state of the TC science. Quoting:

“As pointed out by Emanuel (2001), increased tropical cyclone activity in
a warmer climate would result in increased tropical heat export by the oceans, mitigating tropical warming but amplifying the warming of higher latitudes. This inference is supported by recent numerical simulations using a coupled climate model in which upper ocean mixing is related to a proxy for tropical cyclone activity (Korty et al. 2008). This effect offers a potential explanation for the equable nature of very warm climates, such as that of the early Eocene; high levels of tropical cyclone activity in such warm climates could drive a strong poleward heat flux in the ocean, even in the face of relatively weak pole-to-equator temperature gradients, thus helping to keep such gradients weak. (Todayâ€™s coupled climate models are notoriously bad at reproducing such weak temperature gradients, perhaps because they have no representation of tropical cycloneâ€“induced ocean mixing.) It may also help explain why most of the observed heat uptake by the oceans over the past 50 yr has been in the subtropics and middle latitudes (Levitus et al. 2005), whereas coupled models typically show most of the heat uptake occurring in subpolar regions (e.g. Manabe et al. 1991).”

Well, it turns out there are competing hypotheses, as many as three. Life and nature being complicated, they don’t look to be mutually exclusive. Modeling them may pick a winner, though. My metaphorical money’s on KE.

There is abundant evidence that during periods of equable climate in Earthâ€™s history, such as the Eocene, the interiors of continents at high latitudes were exceptionally warm during winter. Land has a low heat capacity and distances of thousands of kilometers separate some continental interior regions from the ocean. Consequently, significant increases in ocean temperatures do not necessarily imply that temperatures in continental interiors will increase enough to explain proxy data. As a result, explaining winter warmth deep in the interiors of continents remains a central outstanding problem of equable climate dynamics.

Here we present new evidence from global climate models (GCMs) that increases in cloud radiative forcing and the condensation of moisture (convergence of latent heat) over land may play central roles in warming continental interiors at high CO2 concentrations. We show results from both the NCAR CCSM coupled GCM run in Eocene configuration and the NCAR CAM atmospheric GCM run in modern configuration at various CO2 concentrations ranging up to 2240 ppm. The increases in cloud radiative forcing over land may be associated with a novel high-latitude convective cloud feedback that has recently been shown to produce significant warming over the ocean and help reduce sea ice cover at high CO2 levels, and the increased latent heat transport appears to be mainly zonal, rather than meridional.